1. Technical Field
The present invention relates to a gas sensor used for detecting the concentration of a specific gas (gas component) contained in a gas to be measured and a method of manufacturing the gas sensor, and in particular to the gas sensor provided with an oxygen ion conductive solid electrolyte member.
2. Related Art
There has been known a gas sensor for detecting the concentration of a specific gas (i.e., gas component) contained in a gas to be measured, such as an exhaust gas from cars.
This kind of gas sensor is exemplified by EP patent application publication No. 0506897, A gas sensor disclosed by this publication is provided with a cell having electrodes on both sides of an oxygen ion conductive solid electrolyte member and an sensing element integrally having a heater equipped with a heat generating device heating the cell. The sensing element has a base end portion on which there are two pairs of electrode pads electrically connected to the heat generating device and the electrodes, respectively. Practically, the sensing element has two approximately parallel electrode-mounted surfaces, and, of the four electrode pads, two electrode pads are fixedly arranged at two given positions of each electrode-mounted surface, which are aligned in the width direction of each electrode-mounted surface.
The above configuration of the gas sensor is exemplified in FIG. 22, in which there is provided a sensing element 92 having a base end portion 922 at which two electrode-mounted surfaces 923 are formed back to back. As shown, two metal pads 921 are secured on each electrode-mounted surface 923. The metal pads 921 come in contact with metal terminals 932 connected to external lead wires. A pair of base-side insulators 931 has Inner side surfaces to be opposed to each other, on each of which the two metal terminals 932 are secured so as to touch the two metal pads 921 respectively. The base end portion 922 of the sensing element 92 is pinched (gripped) between the two paired base-side insulators 931. A spring member 933, which is shaped into an approximately annular form, is placed to press the paired base-side insulators 931 so that each of the insulators 931 is pressed inwardly. This pressing allows the two pairs of metal terminals 932 are forcibly touched to the two pairs of electrode pads 921.
However, the use of the annular spring member 933 often results in uneven pressing force applied to the one pair of electrode pads 921 arranged in the width direction of each electrode-mounted surface 923. In such a case, it is difficult to give even contact pressure from the four metal terminals 932 to the four electrode pads 921.
In particular, in cases where both electrode-mounted surfaces 923 of the sensing element 92 have a lower parallelism, the metal terminals have irregularities in their thicknesses, or the paired base-side insulators 931 are uneven in their shapes, it is difficult to have even contact pressure of the electrode pads 921 to the metal terminals 932.
Such irregularities in the contact pressure will cause irregularities in contact resistance between each electrode pad 921 and so each metal terminal 932. Thus, detection accuracy of cells in the sensing element 92 may be reduced and/or activation time of the cells may be delayed.